This invention relates to a digital signal playback device such as a compact disk player (referred to hereinafter as CD), a digital audio tape recorder (referred to hereinafter as DAT), a digital video signal recording/playback device (referred to hereinafter as VTR) such as a digital VTR or digital video disk player that records digital video signal and digital audio signal bit streams typified by MPEG2, and more particularly relates to interface control during special playback.
FIG. 37 shows the track pattern of an ordinary home digital VTR. In the figure, slanting tracks are formed on the tape, each track being divided into a video area for recording digital video signals and an audio area for recording digital audio signals.
There are two ways of recording video and audio signals on this home digital VTR. One is baseband recording wherein analog video and audio signals are input and high-efficiency coding is performed on them. The other is transparent recording wherein a digitally transmitted bit stream is recorded.
Transparent recording is more suitable for the Advanced Television now being discussed in the U.S.A. This is because ATV signals are already digitally compressed, do not require a high efficiency coder or decoder, and can be recorded as they are with no picture quality deterioration. The problem however is special playback such as fast, still and slow playback. In particular, almost no pictures are reproduced during fast playback if a bit stream is recorded on slanting tracks without any modification.
One method of using a digital VTR to record these ATV signals is described in a "Method of Recording ATV Data on a Consumer Digital VTR", a technical presentation given at the "International Workshop on HDTV '93" held from Oct. 26-28, 1993 at Ottawa, Canada. This technique will now be described as an example of the prior art.
According to one basic specification of a prototype home digital VTR, in the SD (Standard Definition) mode, one video frame is recorded in video areas of 10 tracks if the recording rate of the digital video signal is 25 Mbps and the field frequency is 60 Hz. Assuming the data rate of an ATV signal is 17-18 Mbps, therefore, ATV signals can be recorded in this SD mode transparently.
FIG. 38A and FIG. 38B show the scanning trace of a rotary head during normal playback and fast playback of a conventional VTR. In the figure, adjacent tracks are recorded slantwise by heads having different azimuth angles. During normal playback, the tape travel speed is the same as in recording, so the head traces the recorded tracks as shown in FIG. 38A. During fast playback however, the tape speed is different so the head cuts across several tracks and can only reproduce fragments of tracks having the same azimuth. FIG. 38B shows the case for 5 times (5.times.) speed fast forward.
In an MPEG bit stream (ATV signals are almost always based on an MPEG2 bit stream), only intra-encoded blocks are decoded independently, without referring to other frames. Assuming the MPEG2 bit stream is sequentially recorded on each track, intra-encoded data will be separated from intermittently reproduced data during fast playback, and the image will be reconstructed only from this separated intra-encoded data. The reproduced area on the screen will be discontinuous, and block fragments will be scattered over the screen. Further, as the bit stream is variable length encoded, there is no guarantee that the whole screen will be periodically updated, and some parts may not be updated for long periods. As a result, the image quality during fast playback is unsatisfactory, and is unsuitable for home digital VTR.
FIG. 39 is a block diagram of a conventional bit stream recording device that permits fast playback. Here, the video area of each track is divided into a main area for recording the whole ATV signal bit stream, and a duplication area for recording important parts of the bit stream (HP data) used in reconstructing the image during fast playback. During fast playback, only intra-encoded blocks are effective so these are recorded in the duplication area, however to further reduce the amount of data, low frequency components are extracted from all intra-encoded blocks and recorded as HP data. In FIG. 39, an input terminal 1 is for receiving the bit stream. An output terminal 2 is for outputting the bit stream. An output terminal 3 is for outputting HP data. Reference numeral 4 is a variable length decoder, 5 is a counter, 6 is a data extracting circuit, and 7 is an EOB (End of Block) appending circuit.
The MPEG2 bit stream is input via the input terminal 1, output via the output terminal 2 and sequentially recorded in the main area. The bit stream from the input terminal 1 is also input to the variable length decoder 4, where the syntax of the MPEG2 bit stream is analyzed, and intra-images are detected, timing is generated by the counter 5. Low frequency components are extracted from all blocks in the intra-images by the data extracting circuit 6, and EOB's are appended by the EOB appending circuit 7 so as to construct HP data which is recorded in the duplication area.
FIG. 40 shows the essential features of a conventional digital VTR during normal playback and fast playback. During normal playback, all of the bit stream recorded in the main area is reproduced and output to the MPEG2 decoder outside the digital VTR. HP data is discarded. On the other hand, during fast playback, only HP data in the duplication area is collected and sent to the decoder, while the bit stream in the main area is discarded.
Next, the arrangement of data in one track of the main area and duplication area will be described. FIG. 41 is a head scanning trace during fast playback. When the tape speed is an integer multiple and phase-locked state is maintained, head scanning is synchronized with tracks having the same azimuth and the positions from which data is reproduced are fixed. In the figure, assuming that useful data is obtained from a part of the playback signal having an output level higher than -6 dB, the hatched areas will be reproduced by one head. FIG. 41 shows the case of 9 times (9.times.) speed, so signal reading of this hatched area is guaranteed at 9 times speed, and HP data may be recorded in this area. At other speeds however, signal reading is not guaranteed. For this reason, the areas where the HP data is recorded must be chosen so that it can be read at several tape speeds.
FIG. 42 shows examples of scanning areas for three tape speeds at which the head is synchronized with tracks having the same azimuth, and overlapping areas (at the bottom of FIG. 42) at the three fast playback speeds (the areas which are reproduced at any of the three fast playback speeds). The duplication area is selected from these overlapping areas, so that reading of HP data at different tape speeds is guaranteed. FIG. 42 shows the case of 4 times (4.times.), 9 times (9.times.) and 17 times (17.times.) speed fast forward, but these scanning areas are the same as for -2, -7 and -15 times (-2.times., -7.times. and -15.times.) speed (i.e., 2, 7 and 15 times reverse speed).
However, it is impossible for the head to trace exactly the same area at several tape speeds due to the fact that the number of tracks the head cuts across depends on the tape speed. Also, the head must be able to trace from any identical azimuth track.
FIG. 43 shows 5 times and 9 times speed head scanning traces in a conventional VTR. In the figure, areas 1, 2, 3 are selected from 5 times and 9 times speed overlapping areas. By repeatedly recording the same HP data on 9 tracks, the HP data can be read at either 5 or 9 times speed.
FIG. 44 shows two head scanning traces at 5 times speed playback in a conventional digital VTR. As seen from the drawing, by repeatedly recording the same HP data on a number of tracks equal to the tape speed, the HP data can be read out by a head synchronized with tracks having an identical azimuth. Therefore, by repeatedly providing HP data on a number of tracks equal to the speed multiplier of the maximum fast playback speed (the speed multiplier being defined by the ratio of the fast playback speed to the normal playback speed), the HP data can be read in the forward or reverse directions at several tape speeds.
FIG. 45 is a track layout diagram in a conventional digital VTR, showing typical main areas and duplication areas. In the home digital VTR, the video area of each track comprises 135 sync blocks, the main area comprises 97 sync blocks and the duplication area comprises 32 sync blocks. For this duplication area, overlapping areas corresponding to the 4, 9, 17 times speeds also in FIG. 42 are selected. In this case, the data rate for the main area is approx. 17.46 Mbps and as the same data is recorded 17 times in the duplication area, the data rate for the duplication areas is approx. 338.8 kbps.
As the conventional VTR has the construction described above, special playback data is duplicated many times in the duplication area, so the recording rate of special playback data is very low and satisfactory image quality cannot be obtained. This is especially true during slow or fast playback. For example, if there are two intraframes per second, the amount of purely intra-encoded data in the ATV signal is estimated to be of approx. 3 Mbps, whereas, in the conventional example, only approx. 340 kbps can be recorded so the picture quality seriously deteriorated.
Where the ATV signal bit stream (transport packets) constructed from data recorded in the special playback area is output during special playback, if for example there is a large amount of data in the intra-frames, the transport packets may overflow in the data transmission process so that the system breaks down at the ATV decoder. Further, the capacity of the special playback memory on the playback side must be extremely large.
FIG. 46A and FIG. 46B show the configuration of error correction codes in the video and audio signal areas in one track of a digital VTR defined in the above SD mode (referred to hereinafter as SD specification). According to the SD specification, error correction codes used in the video signal area are a (85, 77, 9) Reed-Solomon code in the recording direction (referred to hereinafter as C1 check code), and a (149, 138, 12) Reed-Solomon code in the perpendicular direction (referred to hereinafter as C2 check code). Error correction codes used in the audio signal area are a (85, 77, 9) Reed-Solomon code (of the same type as that used for the video signal) in the recording direction (C1 check code), and a (14, 9, 6) Reed-Solomon code in the perpendicular direction (referred to hereinafter as C3 check code). FIG. 47 shows one sync block which is a unit of recording in the recording direction (C1 block). As shown in FIG. 47, one sync block comprises 90 bytes. The sync pattern and ID signal are recorded in the 5 bytes at the head and the error correction code (C1 check code) is recorded in the last 8 bytes.
As described above, the playback signal is reproduced intermittently from the tracks as the rotary head cuts across the recording tracks obliquely during special playback (fast playback, slow playback, still playback). An error correction block (video data) shown in FIG. 46A cannot therefore be constructed during special playback, so only error correction by means of C1 check code can be performed on playback data during fast playback.
If only error correction by means of C1 check code is performed, when the symbol error rate is 0.01, the error detection probability is 1.56.times.10.sup.-3, i.e. one error is detected approximately every 8 sync blocks. Further, as the playback output is not stable during special playback, it often occurs that the symbol error rate is 0.01 or higher. As the recorded data is variable length coded, the subsequent playback data can no longer be used once an error occurs, and this causes deterioration of image quality. Undetected errors also occur very frequently, i.e. 7.00.times.10.sup.-8.